Metals are recognized for their high thermal conductivity relative to various other materials, and are being investigated as heat storage media. In such applications, the metal as the storage medium undergoes successive melting upon heating and freezing upon cooling cycles, and is therefore commonly referred to as a phase charge material. Some metals (and alloys) possess a relatively high latent heat of transformation, and further offer an important advantage for use in heat exchange and storage in that the ratio of heat exchanger area to storage volume can be much smaller, for a given cycle time, than for materials having a poorer thermal conductivity. Additionally, at the melting point of any material, a latent heat of fusion is absorbed. However, at temperatures where latent heat changes could be employed advantageously in heat storage applications, metals generally are not serviceable because they do not retain their shape or rigidity upon melting.
In such a case, an advantageous container for a metal heat storage medium would allow for the transfer of heat between the exterior of the container and the metal, and still retain its mechanical properties despite phase changes (melting and freezing) by the contained metal. Also, an encapsulated phase change material would allow for its direct contact with an energy transporting fluid. A ceramic container, capable of transferring heat to the metal, yet structurally sound enough to contain the metal in service at service temperatures would satisfy these criteria.
U.S. Pat. No. 4,146,057 (granted Mar. 27, 1979 to J. Friedman et al.) discloses an energy storage system for buffering intermittency of and/or asynchronism between an energy supply and energy usage. The energy storage system includes a buffer section comprising a ceramic container filled with aluminum and coupled with a potassium loop and a power and energy output loop. Aside from the generalized statement that a ceramic container is useful for containing aluminum in a heat storage system, there is no disclosure or suggestion as to how the ceramic is produced, let alone being a reaction product layer of the metal.
U.S. Pat. No. 2,823,151 (granted on Feb. 11, 1958 to Yntema et al.) discloses forming a skin comprising an alloy or intermetallic compound on a metal substrate, more particularly a molybdenum metal substrate, in order to render the substrate resistant to oxidation at high temperatures. The skin is described as a molybdenum-silicon-boron alloy or intermetallic compound, and is formed on the molybdenum metal substrate by reacting the underlying molybdenum with silicon and boron, or by plating a non-molybdenum metal substrate with molybdenum, and then reacting the molybdenum with silicon. However, this patent is directed solely to a coating means, and in no way suggests a heat storage medium for the molybdenum is not melted nor does it undergo a melting and freezing transformation. Further, Yntema et al. do not disclose oxidizing the metal base or substrate to produce an encapsulating ceramic container capable of containing the molybdenum metal substrate in a molten state.
British patent application 2,159,542 (filed Mar. 13, 1985 by Zielinger et al.) relates to a method of producing isotropic protective oxide layers on metal surfaces wherein the growth rate of the layer is controlled by varying the oxygen pressure in the growth environment. However, Zielinger et al. do not disclose or suggest growing a ceramic layer of any appreciable strength to contain the coated metal in a molten state nor suggest forming a heat storage medium.
U.S. Pat. No. 4,657,067 to Rapp et al. discloses a thermal storage material utilizing the heat-of-fusion of eutectic alloys such that the outer shell is formed to have a higher melting point than the eutectic core. The material is formed by melting a phase change alloy, and then slowly cooling the melt such that the high melting material present in the gross composition solidifies first and encapsulates the lower-melting, inner eutectic core material.
A novel and useful method for producing self-supporting ceramic bodies by the directed oxidation of a bulk precursor metal (parent metal) is disclosed in the following copending and Commonly Owned Patent Applications. The directed oxidation process lends itself to the process for producing a heat storage medium comprising a self-encapsulating metal.
Accordingly, Commonly Owned U.S. patent application Ser. No. 818,943, filed on Jan. 15, 1986, in the name of Newkirk et al., describes a generic process for producing ceramic materials by the directed oxidation of molten parent metal. In this process, an oxidation reaction product forms initially on the surface of a body of molten parent metal exposed to an oxidant, and then develops outwardly from that surface as molten metal is transported through the oxidation reaction product and into contact with the oxidant at an interface between the oxidant and previously formed oxidation reaction product where it reacts to form a progressively thicker layer of oxidation reaction product. The process may be enhanced by the use of dopants alloyed with the parent metal such as in the case of an aluminum parent metal oxidized in air. This method was improved by the use of dopants applied to the external surface of the parent metal as disclosed in Commonly Owned patent application U.S. Ser. No. 822,999, filed Jan. 17, 1986, in the name of Newkirk et al. In this context, oxidation is considered in its broadest sense to mean one or more metals giving electrons to, or sharing electrons with, another element or combination of elements to form a compound. Accordingly, the term "oxidant" denotes an electron acceptor or sharer
In the process described in Commonly Owned U.S. Ser. No. 819,397, filed Jan. 17, 1986, now allowed by Newkirk et al., ceramic composite products are produced by growing a polycrystalline ceramic product into a bed of filler material adjacent to a body of molten parent metal. The molten metal reacts with a gaseous oxidant, such as oxygen, forming a ceramic oxidation reaction product which permeates the filler. The resulting oxidation reaction product, e.g. alumina, can grow into and through the mass of filler as molten parent metal is drawn continuously through the oxidation reaction product and reacted with the oxidant. The filler particles are embedded within the polycrystalline ceramic product comprising a composite oxidation reaction product. The Commonly Owned Patent Applications do not disclose adapting the directed oxidation process to form a ceramic container around a metal substrate. However, the present invention provides a method for utilizing the directed growth process to develop a ceramic container around a metal body to form a heat storage medium.
Commonly Owned patent application U.S. Ser. No. 861,025, filed May 8, 1986, discloses particularly effective methods in which a filler is formed into a preform with a shape corresponding to the desired geometry of the final composite product. The preform is manufactured by conventional methods to have sufficient shape integrity and green strength, and should be permeable to the transport of oxidation reaction product. Also, an admixture of filler materials and mesh sizes may be used.
Barrier materials may be employed to inhibit or arrest substantially the growth of the oxidation reaction product at a selected boundary to define the shape or geometry of the ceramic structure. This invention was disclosed in Commonly Owned U.S. patent application Ser. No. 861,024, filed May 8, 1986, now allowed in the name of Newkirk et al. and entitled "Method of Making Shaped Ceramic Compositions with the Use of a Barrier".
Commonly Owned patent applications U.S. Ser. No. 823,542, filed Jan. 27, 1986, now allowed and U.S. Ser. No. 896,157, filed Aug. 13, 1986, disclose methods for producing cavity-containing ceramic bodies of a size and thickness which are difficult or impossible to duplicate with previously available technology. Briefly, the inventions therein described involve embedding a shaped parent metal precursor in a conformable filler, and infiltrating the filler with a ceramic matrix obtained by oxidation of the parent metal to form a polycrystalline oxidation reaction product of said parent metal with an oxidant and, optionally, one or more metallic constituents. More particularly, in practicing the invention, the parent metal is shaped to provide a pattern, and then is emplaced in or surrounded by a conformable filler which inversely replicates the geometry of the shaped parent metal. In this method, the filler (1) is permeable to the oxidant when required as in the case where the oxidant is a vapor-phase oxidant and, in any case, is permeable to infiltration by the developing oxidation reaction product; (2) has sufficient conformability over the heat-up temperature interval to accommodate the differential thermal expansion between the filler and the parent metal plus any melting-point volume change of the metal; and (3) when required, at least in a support zone thereof enveloping the pattern, is intrinsically self-bonding, whereby said filler has sufficient cohesive strength to retain the inversely replicated geometry with the bed upon migration of the parent metal as described below. The surrounding or emplaced shaped parent metal is heated to a temperature region above its melting point but below the melting point of the oxidation reaction product to form a molten parent metal. The molten parent metal is reacted in that temperature region or interval with the oxidant to form the oxidation reaction product. At least a portion of the oxidation reaction product is maintained in that temperature region and in contact with and between the body of molten metal and the oxidant, whereby molten metal is progressively drawn from the body of molten metal through the oxidation reaction product, concurrently forming the cavity as oxidation reaction product continues to form within the bed of filler at the interface between the oxidant and previously formed oxidation reaction product. This reaction is continued in that temperature region for a time sufficient to at least partially embed the filler with the oxidation reaction product by growth of the latter to form the composite body having the aforesaid cavity therein. Finally, the resulting a self-supporting composite body is separated from excess filler, if any.
The entire disclosures of each of the foregoing Commonly Owned Patent Applications and Patents, which are assigned to the same owner, are expressly incorporated herein by reference.